1. Field of the Invention
The present invention is generally related to gas pumps and compressors. More particularly, the present invention is related to positive displacement rotary compressors, particularly including those known as Roots blowers and compressors, and to such compressors in such applications as refrigeration, condensation and natural gas transmission.
In particular, the present invention is related to, and constitutes an improvement over, the rotary gas compressors disclosed in the applicant's previously issued U.S. Pat. Nos. 4,859,158 and 5,090,879, issued Aug. 22, 1989 and Feb. 25, 1992, respectively.
2. Description Of Related Art Including Information Disclosed Under 37 CFR 1.97-1.99
The class of positive displacement gas compressors known as Roots compressors, or Roots blowers, has been known and used in industry for over a hundred years. It is well recognized in industry that for certain applications Roots compressors offer a number of advantages over other types of gas pumps and compressors, for example conventional piston-and-cylinder reciprocating pumps, fan-type blowers and turbine pumps. Among these advantages are simplicity, ruggedness, trouble-free operation, and high volumetric capacity. Roots compressors have no valves, pistons or other reciprocating mechanical parts. Additionally, Roots compressors have little or no backflow, even when the compressor is not operating. A typical application of a Roots compressor is the transfer or evacuation of large amounts of toxic or corrosive gas, where it is important to rapidly pump large amounts of gas with little or no backflow. In this type of application reciprocating pumps are relatively inefficient, and fan-type blowers and turbine pumps cannot provide a seal against backflow.
Roots compressors most commonly include two lobed impellers, sometimes also called rotors, which intermesh with one another and rotate in opposite directions in synchronization within a housing. The impellers operate to sweep a gas through the housing from an intake manifold at one end of the housing to an output manifold at the opposite end of the housing. Commercially available Roots compressors most commonly include impellers having only two lobes. However, Roots compressors have also been designed to include impellers having three, four and even more lobes. Two-lobed impellers are the most common, however, for several reasons. One reason is that they are simpler to construct and maintain. Also, they are characterized by a relatively higher volumetric efficiency. This high efficiency is due to the fact that the volumetric efficiency of a Roots compressor is generally inversely proportional to the proportion of the compressor chamber that is occupied by the impellers; and two-lobed impellers generally occupy a smaller volume than impellers having more lobes.
Roots compressors are extraordinarily efficient for the purpose of rapidly moving large volumes of gas where there is a relatively small pressure gradient across the compressor. Roots compressors have not heretofore been of useful application for the purpose of pumping a gas against a substantial pressure differential. This limitation has been due to heating effects which attend such pumping. As a gas is swept through a conventional Roots compressor from a region of relatively low pressure to a region of relatively higher pressure, it is compressed and heated. Such compression is essentially adiabatic, such that the temperature of the gas increases exponentially with increasing pressure ratios. The increase in the temperature of the gas leads to heating of the impellers, the housing and the other mechanical parts of the pump. This in turn can lead to thermal distortion, expansion and friction. At pressure ratios of greater than about two to one (2:1) such effects become a significant problem and essentially limit the sustained capacity of the compressor. Overheating of the compressor can result in lockup or other mechanical failure of the impellers, seals and other compressor components.
This heating problem is not uniform throughout the compressor. The compressor housing, for example, can be externally cooled by a number of conventional methods, such as the use of integral double-walled water jackets, heat radiating fins, heat sinks, and the like. The greatest heating problem however lies with the impellers, because there is no practical way to directly cool the impellers. Overheating of the impellers leads to their expansion and eventual binding against the housing, possibly causing extensive damage to the compressor. Overheating of the Roots compressor has thus been one of the major limitations on the use of Roots compressors for pumping gas against high pressure differentials, and for this reason commercially available Roots compressors are typically limited to pressure ratios of less than about four to one (4:1).
Perhaps the most simple and straightforward method of avoiding the adverse effects of overheating is to increase the clearances between the impellers and the housing, thereby allowing the impellers to expand somewhat on heating without rubbing and locking up against the housing. This however necessarily leads to increased gas leakage and backflow, and thereby degrades the volumetric efficiency of the compressor. For this reason, this approach has not generally been considered a satisfactory solution to the overheating problem.
A substantial advance in the art was the development of recirculation cycles to effect a moderate reduction in the heating of Roots compressors. In a recirculating Roots compressor, a portion of the output gas, which is compressed to a higher pressure than the input gas, is recirculated back into the compressor so as to effectively increase the pressure of the gas passing through the compressor. In some recirculating compressors a portion of the output gas is cooled prior to being recirculated back into the compressor. In both cases the operating temperature of the compressor is effectively reduced, thereby mitigating the overheating problems referred to above.
U.S. Pat. No. 2,489,887 to Houghton, for example, discloses the general concept of cooling the impellers of a Roots compressor by introducing recirculated gas of a lower temperature into the intake gas to reduce heating of the impellers.
U.S. Pat. No. 3,351,227 to Weatherston discloses a multi-lobed Roots-type compressor having feedback passages which allow a portion of the high-pressure discharge gas to be recirculated back into the pump housing. Weatherston however discloses only the use of quite small feedback passages, the size of which are unrelated to the sizes of the intake and discharge ducts. This results in uneven flow velocities and pressures. As will be apparent from the description of the present invention set forth below, this does not solve the flow problems addressed by the present invention.
German Patent 2,027,272 to Kruger discloses the concept of cooling and recirculating discharge gas in a two-lobed Roots compressor. The compressor of Kruger, due to its two-lobed configuration, has no provision for preventing backflow from the discharge port into the recirculation ports.
French Patent 778,361 to Bucher discloses four-lobed Roots compressors having recirculation ports. The recirculation ports are however small, with the intended purpose of using small nozzle-like ports being to allow the recirculated gas to adiabatically cool upon entry into the compressor housing. As Will be apparent from the description below, this teaching of Bucher is contrary to the present invention.
U.S. Pat. No. 4,453,901 to Zimmerly discloses a positive displacement rotary pump which is designed for pumping liquids, and which contains no provision for recirculation.
U.S. Pat. No. 4,390,331 to Nachtrieb discloses rotary compressor having four-lobed impellers, but likewise having no provision for recirculation.
U.S. Pat. No. 2,906,448 to Lorenz discloses a rotary positive displacement compressor having two-lobed impellers, with a double-walled construction for cooling purposes.
British Patent 282,752 to Kozousek discloses a rotary pump which is characterized by rotor lobes which are particularly shaped so as to provide the maximum possible working space and thereby maximize the volumetric efficiency of the pump. The pump disclosed in Kozousek discloses recirculation ports which are deliberately made small, and which are for the purpose of obtaining even delivery of gas.
Various kinds of Roots compressors are commercially available, both with and without recirculation. However, none of the commercially available compressors address the problems of recirculation flow impedance and flow velocity equalization which are addressed by the present invention.
In some prior art recirculating Roots compressors, such as the compressor described in Houghton, the flow of recirculating gas is either periodically interrupted each time a rotor lobe passes the recirculation entry port, or is halted and possibly even reversed as a displacement cavity is simultaneously opened to both a recirculation port and a discharge port. This results in a loss of momentum and flow of the recirculation fluid, reducing the efficiency of the recirculation fluid in cooling the compressor. This problem, which is inherent in many previously known Roots compressors, is overcome in the present invention, as will be made apparent by the descriptions set forth below.
In the applicant's previously issued U.S. patents, cited above, certain improvements were disclosed which achieved lower operating temperatures by recirculation of the working fluid. The present invention provides certain improvements in the compressors described in those Patents.
Accordingly, it is the object and purpose of the present invention to provide an improved positive displacement rotary gas compressor.
It is also an object and purpose of the present invention to provide a positive displacement rotary gas compressor having an improved gas recirculation means for reducing overheating of the compressor.
It is a further object and purpose of the present invention to provide a positive displacement rotary gas compressor which is characterized by having a continuous, uninterrupted flow of recirculation fluid which flows from the output of the compressor back into the compressor.
It is also an object and purpose of the present invention provide a positive displacement rotary gas compressor that produces less heat inside the compressor and is thus capable of operating at higher pressure ratios than have been previously available.
It is also an object of the present invention to provide a positive displacement rotary gas compressor that is particularly suited for use in combination with a vapor condenser, for example for compressing condensable gases as in a refrigeration apparatus.
It is yet another object of the present invention to provide a rotary gas compressor which utilizes flow work to achieve improved efficiency through substantially isothermal gas compression.
It is yet another object of the invention to attain the foregoing objects and also reduce acoustic noise emission from the compressor.